Compact objective lens assembly for simultaneously imaging multiple spectral bands

ABSTRACT

An objective lens assembly suitable for use in helmet-mounted applications. The objective lens assembly comprises two prisms that collectively are configured, oriented and bonded relative to each other to separate and allow simultaneous imaging of two separate spectral bands (such as VNIR and LWIR bands) received from the same object scene via a common window such that the object scene may be viewed from the same perspective without the effects of parallax.

FIELD OF THE INVENTION

The present invention relates to image-forming optical systems forapplications in the visible, near infrared and thermal infrared regionsof the electromagnetic spectrum. In particular, the present inventionrelates to a compact objective lens assembly having a single entrancepupil and two prisms arranged in an angular relationship to one anothersuch that two or more spectral bands received though the entrance pupilare separated by the two prisms for simultaneous imaging and the opticalpath associated with each spectral band conforms to a surface of ahelmet or other headgear.

BACKGROUND OF THE INVENTION

With the advent of small, low power stand-alone electronic type camerasfor the detection of visible, near-infrared (VNIR) and long-waveinfrared (LWIR) images (such as the conventional image-intensified CMOSimager for VNIR image detection and the uncooled micro-bolometer focalplane array imager for LWIR image detection), the number of applicationsfor these types of VNIR or LWIR cameras has expanded to includehand-held, weapon-borne and helmet-mounted applications. However, theconventional VNIR and LWIR cameras each have an image-forming opticalsystems or objective lens assembly that is typically too long in afield-of-view axis to be suitable for use in hand-held, weapon-borne andhelmet-mounted applications.

The present inventor has disclosed, in U.S. patent application Ser. No.11/539,804, a dual-field-of-view objective lens assembly that is compactand thin in the object side direction such that the objective lensassembly is suitable for use in hand-held, weapon-borne andhelmet-mounted applications. However, the disclosed dual-field-of-viewobjective lens assembly requires separate prism lens groupings withrespective entrance pupils for viewing two spectral band images, such asVNIR and LWIR images. Thus, there is a need for a compact lens assemblythat employs fewer optical components for simultaneously imaging two ormore spectral band images, for example, within the VNIR waveband and theLWIR waveband.

U.S. Pat. No. 7,248,401 (the “'401 patent”) discloses a common-aperture,multispectral objective device that uses a folded beam-splitter andmirror to simultaneously image near infrared (NIR) and LWIR spectralbands. However, the objective device disclosed in the '401 patent whenimplemented for demonstration to the US Army was found to be susceptibleto producing an undesirable forward projection. In addition, the '401patent's objective device is neither compact in the object direction norform-fitting to a helmet or other headgear.

Conventional color separating prism assemblies, such as disclosed inU.S. Pat. Nos. 6,667,656; 6,517,209; and 6,078,429, also employ a commonaperture for receiving an incoming light beam from an objective lensassembly. However, conventional color separating prism assemblies arenot suitable for use as an objective lens or for separating spectralbands within the infrared band for simultaneous imaging. In addition,conventional color separating prisms are neither compact in the objectdirection nor form-fitting to a helmet or other headgear.

Therefore, a need exists for an objective lens assembly that overcomesthe problems noted above and others previously experienced forsimultaneously imaging multiple spectral bands, for example in the VNIRand the LWIR bands, for use in hand-held, weapon-borne andhelmet-mounted applications.

SUMMARY OF THE INVENTION

Optical systems and assemblies consistent with the present inventionprovide a compact objective lens assembly suitable for use inhelmet-mounted applications. The objective lens assembly is configuredto allow simultaneous imaging of two separate spectral bands (such asVNIR and LWIR bands) received from the same object scene via a commonwindow such that the object scene may be viewed from the sameperspective without the effects of parallax.

In one embodiment, the objective lens comprises a first prism and asecond prism. The prisms are configures to effectively enable theprincipal ray optical path of a first band (e.g., LWIR band) through thefirst prism and a principal ray optical path of a second band (e.g.,VNIR band) through the first prism and the second prism so that eachoptical path conforms to a compound surface of a helmet. The first prismhas, in sequence of light propagation from an object, a firsttransmitting surface A defining an entrance pupil of the objective lensassembly, a partial reflecting surface B and a second transmittingsurface C. The first transmitting surface A is operatively configured topass light from the object to the partial reflecting surface B. Thepartial reflecting surface B is operatively configured to reflect afirst portion of the passed light associated with a first of a pluralityof infrared spectral bands back towards the first transmitting surface Aso that the reflected first portion of the passed light is totallyinternally reflected (TIR) by the first transmitting surface A towardsthe second transmitting surface C. The second transmitting surface C isoperatively configured to allow the reflected first portion of thepassed light from the first transmitting surface A to pass through andexit the first prism. The partial reflecting surface B is furtherconfigured to transmit a second portion of the passed light associatedwith a second of the infrared spectral bands towards the second prism.The second prism has, in sequence of light propagation from the object,a first transmitting surface D, a reflecting surface E and a secondtransmitting surface F. The first transmitting surface D is operativelyconfigured to pass the second portion of light exiting the partialreflecting surface B of the first prism to the reflecting surface E ofthe second prism. The reflecting surface E is operatively configured toreflect the second portion of passed light back towards the firsttransmitting surface D so that the reflected second portion of thepassed light is totally internally reflected (TIR) by the firsttransmitting surface D towards the second transmitting surface F. Thesecond transmitting surface F is operatively configured to allow thereflected second portion of the passed light from the first transmittingsurface D to pass through and exit the second prism.

In one implementation, the first prism has a dichroic coating thatdefines the partial reflecting surface B of the first prism and isadapted to reflect the first portion and transmit the second portion ofthe passed light. In this implementation, the first portion of thepassed light is within the long wave infrared band and the secondportion of the passed light is within the near-infrared band.

Other systems, methods, features, and advantages of the invention willbecome apparent to one with skill in the art upon examination of thefollowing figures and detailed description. It is intended that all suchadditional systems, methods, features, and advantages be included withinthis description, be within the scope of the invention, and be protectedby the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate implementations of the inventionand, together with the description, serve to explain the advantages andprinciples of the invention. In the drawings,

FIG. 1 is a perspective side view of an exemplary imaging device forsimultaneously imaging two separate wavebands emanating from an objectin accordance with the present invention, where the imaging device ismounted on a helmet;

FIG. 2 is a perspective front view of the imaging device mounted on thehelmet;

FIG. 3 is a perspective top view of the imaging device mounted on thehelmet;

FIG. 4A is a sectional view of one embodiment of an objective lensassembly of the imaging device consistent with the present invention,where the objective lens assembly includes a first prism and a secondprism operatively configured for simultaneously separating a specularlight waveband and emitted light waveband emanating from the object andentering a first transmitting surface of the first prism;

FIG. 4B is a magnified sectional, cutaway view of a bonding layerattaching a partial reflecting surface of the first prism operativelyconfigured to reflect the emitted light waveband towards the firsttransmitting surface of the first prism and a transmitting surface ofthe second prism operatively configured to receive and internallytransmit the specular light waveband that passes through the partialreflecting surface of the first prism and the bonding layer to thetransmitting surface of the second prism;

FIG. 5 depicts a prism corresponding to the first prism in FIG. 4A, andillustrates four significant variables used to derive the internal apexangles of the first prism and the corresponding internal apex angles ofthe second prism; and

FIG. 6 is a sectional top view of the compact lens assembly, oneembodiment of a LWIR camera of the imaging device and one embodiment ofa VWIR camera of the imaging device arranged for mounting on the frontof the helmet in accordance with the present invention such that theLWIR camera receives the emitted light waveband from the first prism andthe VNIR camera receives the specular light waveband from the secondprism.

DETAILED DESCRIPTION OF THE INVENTION

Optical systems and imaging assemblies consistent with the presentinvention provide a compact objective lens assembly suitable for use inhelmet-mounted applications. The objective lens assembly is configuredto provide simultaneous imaging of two separate spectral bands (such asvisible-near infrared (VNIR) and long wave infrared (LWIR)) so that theoptical path of each respective spectral band through the objective lensassembly conforms to a compound surface of a helmet while enabling theobjective lens assembly to be compact in size.

FIGS. 1, 2 and 3 are sectional side, front and top views, respectively,of one embodiment an imaging device 100 consistent with the presentinvention for simultaneously imaging two separate wavebands emanatingfrom an object 1. The imaging device 100 comprises an objective lensassembly 101 mounted on a compound surface 3 of a helmet 5. Theobjective lens assembly 101 includes a first prism 102 and a secondprism 104 operatively configured to collectively function as a wavebandseparating means to separate a first light waveband or emitted lightwaveband (e.g., LWIR band), and a second light waveband or reflectedspecular light waveband (e.g., VNIR band) emanating from an object 10 orscene. The combined wavebands are received by the objective lensassembly 101 via a common entrance pupil, which in the implementationshown in FIGS. 1-3 is a first transmitting surface A (106) of the firstprism 102. The respective wavebands are subsequently separated via theprisms 102 and 104 and transmitted through a respective exit pupil(e.g., transmitting surfaces 110 or 116 as best shown in FIG. 4A) to acorresponding objective lens entrance pupil plane (e.g., pupil plane 118or 120 as best shown in FIG. 4A) for capture by a respective camera(e.g., LWIR camera 122 and VNIR camera 124) and processing by a commonbackend processor 126 as described in further detail below.

Each camera 122 and 124 has a respective waveband objective lensassembly 128 or 130 coupled to or incorporating the respective objectivelens entrance pupil plane 118 or 120 and a respective pixel focal planearray 130 or 132 for capturing the respective wavebands separated by theprisms 102 and 104. In the implementation shown in FIGS. 1-3, theobjective lens assembly 101 is arranged in combination with each camerawaveband lens assembly 128 and 130 and corresponding pixel focal planearray 130 or 132 so that each combination of camera waveband lensassembly 128 and 130 and corresponding pixel focal plane array 130 and132 are disposed along and conform to the compound surface 3 of thehelmet 5. This arrangement enables the imaging device 100 to be compactin size and have a low projection profile with respect to the helmet 5.

For clarity in the discussion to follow, the objective lens assembly 101is oriented in reference to the object 5 or scene (from which thecombined wavebands are reflected or are emitted), which has a bodycoordinate system 10. The body coordinate system 10 of the object (andthe objective lens assembly 101) has a Z-axis 12, a Y-axis 14 (into thepage in FIG. 2), and a X-axis 16 (into the page in FIG. 1). A firstaxial principal ray 20 received by the objective lens assembly 101 isdefined as an emitted ray of the first light waveband (e.g., LWIR)emerging from the center of the object 5 that is incident on theentrance pupil or first transmitting surface 106 of the first prism.Another axial principal ray 30 received by the objective lens assembly100 is defined as a ray of the second light waveband (e.g., VWIR)emerging from the center of the object that is incident on the entrancepupil or first transmitting surface 106 of the first prism. The axialprincipal rays 20 and 30 define the Z-axis 12 of the body coordinatesystem 10 for the object and the respective objective lens assembly 101.The axial principal rays 20 and 30 are shown spaced apart in FIG. 1, butmay be received on the entrance pupil 106 of the objective lens assembly101 as a combined signal or sequential signals on the same Z-axis 12 asshown, for example, in FIG. 4A.

The direction in which the Z-axis 12 extends from the object to theentrance pupil or the first surface (e.g., surface 106) of the objectivelens assembly 101 is defined as positive. The body coordinate system 10follows the known right hand rule with reference to the axial principalrays 20 and 30 to define the Y-axis 14 of the object and the objectivelens assembly 101 as being orthogonal and vertical to the Z-axis 12. TheX-axis 16 is orthogonal to both the Z-axis 12 and the Y-axis 14. In thevarious illustrations given for each embodiment or example of anobjective lens assembly consistent with the present invention, thedirection of light propagation is initially from along the positiveZ-axis 12. Reflecting surfaces of the objective lens assembly 101 maysubsequently reverse the direction of the light propagation. Therefore,after an odd number of reflections, the Z-axis 12 will be negative.

As discussed below, the axial principal ray 20 of the first band(referenced hereafter as the “LWIR principal ray 20”) forms a respectiveoptical path through the first prism 102 and subsequently through theLWIR camera objective lens assembly 128 to the pixel focal plane array132 while the axial principal ray 30 of the second band (referencedhereafter as the “VNIR principal ray 30”) forms a respective opticalpath through the first prism 102 and the second prism 106 andsubsequently through the VNIR camera objective lens assembly 130 to thepixel focal plane array 134.

FIG. 4A depicts a sectional view of one embodiment of the objective lensassembly 101 of the imaging device 100 consistent with the presentinvention. As shown in FIG. 4A, the first prism 102 has, in sequence oflight propagation from the object, a first transmitting surface A 106defining the entrance pupil of the objective lens assembly 101, apartial reflecting surface B 108 and a second transmitting surface C110. The first transmitting surface A 106 is operatively configured topass light (including both the LWIR principal ray 20 and the VNIRprincipal ray 30) from the object to the partial reflecting surface B108. The partial reflecting surface B 108 is operatively configured toreflect a first portion of the passed light (e.g., ray 30′ in FIGS. 4Aand 4B) associated with a first of a plurality of infrared spectralbands (e.g., the LWIR band) back towards the first transmitting surfaceA 106 so that the reflected first portion of the passed light is totallyinternally reflected (TIR) by the first transmitting surface A 106towards the second transmitting surface C 110. The second transmittingsurface C 110 is operatively configured to allow the reflected firstportion of the passed light from the first transmitting surface A 106(e.g., the LWIR band) to pass through and exit the first prism 102towards the objective lens entrance pupil plane 118. The partialreflecting surface B 108 is further configured to transmit a secondportion of the passed light (e.g., ray 20 in FIGS. 4A and 4B) associatedwith a second of the infrared spectral bands (e.g., the VNIR band)towards the second prism 104.

The second prism 104 has, in sequence of light propagation from theobject, a first transmitting surface D 112, a reflecting surface E 114and a second transmitting surface F 116. The first transmitting surfaceD 112 is operatively configured to pass the second portion of light(e.g., the VNIR band) exiting the partial reflecting surface B 108 ofthe first prism 102 to the reflecting surface E 114 of the second prism104. The reflecting surface E 114 is operatively configured to reflectthe second portion of passed light back (e.g., the VNIR band) towardsthe first transmitting surface D 112 so that the reflected secondportion of the passed light (e.g., ray 20) is totally internallyreflected by the first transmitting surface D 112 towards the secondtransmitting surface F 116. To reflect the second light band (e.g., VNIRband) back towards the first transmitting surface D 112 of the secondprism 104, the reflecting surface E 114 of the second prism 104 may havea metallic or dielectric mirror coating. The second transmitting surfaceF 116 is operatively configured to allow the reflected second portion ofthe passed light from the first transmitting surface D 112 (e.g., thereflected VNIR band) to pass through and exit the second prism 104towards the objective lens entrance pupil plane 120.

In one implementation, the first prism 102 may be constructed from athermal infrared transmitting material having a refractive index (n₂),at a wavelength of 10 micrometers, greater than 2.3. The second prismmay be constructed from a thermal infrared transmitting material havinga refractive index (n₄), at a wavelength of 0.85 micrometers, greaterthan 2.0. In this implementation, the first prism 102 and the secondprism 104 may each comprise or be formed of an optical material capableof transmitting the near-infrared (NIR) and thermal infrared regions ofthe electromagnetic spectrum. For example, each of the prisms 102 and104 may comprise or be formed of zinc selenide or zinc sulfide. Inanother implementation, prism 104 may comprise or be formed of anoptical glass (such as glass type S-LAH79, commercially available fromthe OHARA Corporation).

In one implementation, a short-pass dichroic coating 402 is deposited onthe surface B 108 as shown in FIG. 4B to define or create the partiallyreflecting surface B 108 and enable the partially reflecting surface B108 to reflect the first light waveband (e.g., LWIR band wavelengthsreflected by the LWIR principal ray 30 and other LWIR rays 30 b and 30 cincident on the entrance pupil 106) that pass through the entrance pupil106 and are incident on the partial reflecting surface B. The short-passdichroic coating 402 is configured to transmit the second light waveband(e.g., VNIR band wavelengths reflected by the VNIR principal ray 20)that has shorter wavelengths then the first light waveband to passthrough the dichroic coating 402 towards the first transmitting surfaceD 112 of the second prism 104. In one implementation, the dichroiccoating 402 is adapted reflect a wavelength within a range of 8 μm to 12μm corresponding to the first light waveband (e.g., the LWIR band) andto transmit a wavelength within a range of 0.6 μm to 0.9 μmcorresponding to the second light waveband (e.g., VNIR band).

In one implementation, the objective lens assembly 101 includes abonding layer 404 disposed between the partial reflecting surface B 108of the first prism 102 and the first transmitting surface D 112 of thesecond prism 104. The bonding layer 404 comprises an optical bondingmaterial or cement epoxy that transmits a wavelength within the secondlight waveband. In the implementation in which the dichroic coating 402transmits light within the near-infrared (NIR) band, the bonding layer404 comprises an optical bonding material (such as NOA-61 or NOA-71commercially available from Norland Co., Ltd), that transmits awavelength within the near-infrared band. A ratio (n₃/n₂) of arefractive index (n₃) of the bonding layer to a refractive index (n₃) ofthe first prism is equal to or greater than 0.650. The bonding layer 402has a thickness (t) preferable within the range of 0.20 mm to 0.30 mm.Since the spacing between the first prism's partial reflecting surface B108 and the first transmitting surface D 112 of the second prism 104 isfilled in this implementation with the bonding layer 404, the bondinglayer 404 prevents ghost images from appearing in either camera imageplane 118 or 120 due to light from the second waveband exiting the firstprism 102 via the partially reflecting surface B 108 and being refractedor reflected by the surfaces on either side of the air gap between thetwo prisms 102 and 104.

In one implementation, the first prism 102 has internal apex angles ofapproximately 27.5° (406 in FIG. 4A), 55° (408), and 97.5° (410). Inthis implementation, the second prism 104 has internal angles ofapproximately 47.5° (412 in FIG. 4A), 38° (414), and 94.5° (416). Inthis configuration, the objective lens assembly 101 has a thickness (d)along the Z-axis 12 of 25 millimeters or less. Thus, the projectionprofile of the objective lens assembly (and, thus the imaging device100) relative to the helmet 5 is less than other conventional helmetmounted imaging devices or cameras. In this implementation, the entrancepupil 118 of the LWIR camera 122 is approximately parallel to the secondtransmitting surface C 110 of the first prism 102 such that the entrancepupil plane 118 is disposed at an angle of 56 degrees or more from theZ-axis 12 or central axis of the helmet 5, allowing the LWIR camera 122to be disposed along the compound surface 3 of the helmet 5 in adirection from the front of the helmet where the objective lens assembly101 is disposed towards the rear of the helmet 5. Similarly, in thisimplementation, the entrance pupil plane 120 of the VNIR camera 124 isapproximately parallel to the second transmit surface F 116 of thesecond prism 104 such that the entrance pupil 120 is disposed at anangle of 75 degrees or more from the Z-axis 12 or central axis of thehelmet 5, allowing the VNIR camera 124 to be disposed on the oppositeside of the helmet 5 from the LWIR camera 122 along the compound surface3 of the helmet 5 in a direction from the front of the helmet 5 wherethe objective lens assembly 101 is disposed towards the rear of thehelmet 5.

FIG. 5 depicts a prism 500 corresponding to the first prism 102 in FIG.4A, and illustrates four significant variables used to derive theinternal apex angles of the first prism 102 and the correspondinginternal apex angles of the second prism 102. The four significantvariables illustrated in FIG. 5 are the incident cone angle (A1), theprism angle (A4) (which corresponds to the first internal apex angle 406of the first prim in FIG. 4A), prism index (n2), and bonding layer orcement index (n3).

Assuming a collimated, orthogonal incident ray on the entrance pupil,the internal angle with respect to the prism hypotenuse (A2 b) is theinternal apex angle, A4, of the prism 500. If a cone angle is introduced(e.g., a LWIR or VNIR light ray enters the entrance pupil at the coneangle A1), then the most extreme ray angle (A1) may be used to describethe worst-case transmission and reflection angles through the prism 500.Depending on the index of the prism 500, the angle of this ray isreduced (assuming an incident medium of air, n₁=1) according to equation(1).

$\begin{matrix}{{A\; 2a} = {a\;{\sin\left( {\frac{1}{n_{2}}{\sin\left( {A\; 1} \right)}} \right)}}} & (1)\end{matrix}$

This ray is transmitted through the medium of the prism 500 until itencounters the exit surface (which corresponds to the partial reflectingsurface 108 of the first prism 102), where the incident angle, A2 b,must be redefined with respect to the hypotenuse (A4) as reflected inequation (2).A2b=A2a+(90−A4)  (2)

Angle A2 a is increased by the factor 90−A4, where A4=90 degrees for twoparallel surfaces (for manufacturing purposes, specified prism angle isin fact 90−A4, using the conventions in FIG. 5). As angle A2 bincreases, a critical angle for internal reflection is approached. At orabove the critical angle, transmission occurs parallel to the exitsurface (90 degrees), and all rays are instead internally reflected.Assuming A3=90 degrees, the critical angle (θ_(c)) for angle A2 b can bedefined according to Snell's Law reflected in equation (3) below.

$\begin{matrix}{\theta_{c} = {a\;{\sin\left( \frac{n_{3}}{n_{2}} \right)}}} & (3)\end{matrix}$

Assuming the prism 500 is comprised of zinc selenide (n₂≈2.5 for visiblewavelengths) and the bonding layer is comprised of Norland 61 epoxy(n₃≈1.56), then the critical angle (θ_(c)) is approximately equal to38.6 degrees. For a system with a maximum half angle of 20 degrees (40degree field of view) and prism internal apex angle (A4) of 28 degrees,the incident angle for the exit surface (A2 b) is approximately 35.9degrees in accordance with equation (2). In this implementation,transmission occurs only when the prism angle (A2 b) is below 30 (e.g.,A4=60) degrees. For collimated light, this maximum angle becomesapproximately 38 (A4=52) degrees. As previously discussed, in apreferred implementation, the prism internal apex angle (A4) of prism102 is 27.5 degrees.

The indices of both the material used to form the first and secondprisms 102 and 104 as well as the bonding layer 404 material affect thedetermination of the critical angle for A2 b (when total internalreflection of rays incident on the exit surface occurs rather thantransmission) and, as well as, the determination of the exit ray angles.As shown previously in equation 2, the ratio of bonding layer materialor epoxy index to prism index will drive the critical angle. Increasingthe refractive index of the bonding layer material or decreasing theprism indices would both serve to increase the critical angle, and thusthe range of available prism and cone angles. Since most optical epoxiesare limited to index values under 1.57, the prism index becomes thevariable of choice. For example, zinc sulfide (n₂≈2.3) increases thecritical angle to approximately 42 degrees. In another implementation,the prisms 102, 106 and 500 may comprise fluoride materials (e.g.,calcium fluoride or barium fluoride) that transmit well in NIRwavelength regions, but are significantly less desirable due to theircoating adhesion properties, thermal stability, and tensile strength.

As shown in FIG. 4A, the waveband separating surfaces of the first andsecond prisms 102 and 104 are connected throughout in an unbrokensequence, such that double images and the concomitant deterioration inimage quality associated with double images are prevented. Moreover,since the two prisms 102 and 104 are operatively configured to receiveand separate two light wavebands (e.g., LWIR and VNIR bands) emanatingfrom the same object 50 or scene via a common entrance pupil 108, theimages captured at the respective camera image plane 118 or 120 may beprocessed by the back-end processor 126 and displayed on a display (notshown in figures) without the effects of parallax that result fromviewing an object scene using two cameras with separate entrance pupilsand objective lenses.

FIG. 6 is a sectional top view of the compact lens assembly 101, oneembodiment of the LWIR camera 122 of the imaging device 100 and oneembodiment of a VNIR camera 124 of the imaging device 100 arranged formounting on the front of the helmet 5 in accordance with the presentinvention such that the LWIR camera 122 receives the first lightwaveband (e.g., the object's emitted light waveband) from the firstprism 102 and the VNIR camera 124 receives the second light waveband(e.g., the object's reflected specular light waveband) from the secondprism 104. In the implementation shown in FIG. 6, the waveband objectivelens assembly 128 of the LWIR camera 122 includes, in sequence of lightpropagation from the exit or second transmitting surface C 110 of thefirst prism 102 or from the entrance pupil plane 118 of objective lensassembly 128, the following optical elements: a negative power meniscuslens 602 having a first surface 604 and a second surface 606 (each ofwhich may be spherical or employ an asphero-diffractive surface orkinoform surface); a power lens 608 having a front convex surface 610and a rear surface 612 that may be spherical or planar; a positive powerlens 614 (which may be a positive meniscus lens); and a plane parallelplate 618, which represents the window of the LWIR camera 122 and allowstransmission of the second light waveband towards the pixel focal planearray 132. The plane parallel plate 618 and the pixel focal plane array132 may be incorporated into the objective lens assembly 128 of the LWIRcamera 122.

The pixel focal plane array 132 may include an array of micro-bolometerpixels adapted to capture an LWIR image corresponding to the first lightwaveband (as emitted from the first prism exit surface C 110) that isreceived by the pixels or micro-bolometers over an integration periodcontrolled by the back-end processor 126 using standard imagingtechniques.

In the implementation shown in FIG. 6, the waveband objective lensassembly 130 of the VNIR camera 124 includes, in sequence of lightpropagation from the exit or second transmitting surface F 116 of thesecond prism 104 or from the objective lens entrance pupil plane 120,the following optical elements: a positive power lens 620 facing theexit surface F 116 of the second prism 104; a first achromatic doublet622 having a negative power lens 624 and a positive power lens 626 wherethe front surface 628 of the negative power lens 624 faces the positivepower lens 620 and the rear surface 630 of the positive power lens 626faces away from the lens 624; a second achromatic doublet 632 having apositive power lens 634 and a negative power lens 636 where the frontsurface 638 of the positive power lens 634 faces the first achromaticdoublet 622 and the rear surface 640 of the negative power lens 636faces away from the lens 634; and a plane parallel plate or window 644.The plate or window 644 represents the window of the VNIR camera 124. Inthe implementation shown in FIG. 6, the plane parallel plate or window644 has a rear surface that corresponds to the pixel focal plane array134 of the VNIR camera 124. The plane parallel plate or window 644 maybe incorporated into the objective lens assembly 130 of the VNIR camera124.

The pixel focal plane array 134 may include an array of pixels (such asmicro-bolometers or photodetectors) adapted to capture a VNIR imagecorresponding to the second light waveband (as emitted from the secondprism exit surface F 116) that is received by the pixels over anintegration period controlled by the back-end processor 126 usingstandard imaging techniques. The back-end processor 126 may beconfigured to synchronize the trigger and control of the LWIR camera 122and the VNIR camera 124 such that each of the pixel focal plane arrays132 and 134 simultaneously capture a respective image (i.e., a LWIRimage and a VNIR image) from the same object 5 scene as viewed throughthe same entrance pupil 106 of the objective lens assembly 101. Theback-end processor 126 may also be configured to provide thesimultaneously captured LWIR image and VNIR image to a display (notshown in figures) so that a user may view the two images of the sameobject 5 scene without the effects of parallax.

While various embodiments of the present invention have been described,it will be apparent to those of skill in the art that many moreembodiments and implementations are possible that are within the scopeof this invention. Accordingly, the present invention is not to berestricted except in light of the attached claims and their equivalents.

1. An objective lens assembly suitable for simultaneously imaging aplurality of infrared spectral bands, comprising: a first prism and asecond prism disposed along a central axis of the objective lensassembly, the first prism having, in sequence of light propagation froman object, a first transmitting surface A defining an entrance pupil ofthe objective lens assembly, a partial reflecting surface B and a secondtransmitting surface C, the first transmitting surface A beingoperatively configured to pass light from the object to the partialreflecting surface B, the partial reflecting surface B being operativelyconfigured to reflect a first portion of the passed light associatedwith a first spectral band of the plurality of infrared spectral bandsback towards the first transmitting surface A so that the reflectedfirst portion of the passed light is internally reflected by the firsttransmitting surface A towards the second transmitting surface C, thesecond transmitting surface C being operatively configured to allow thereflected first portion of the passed light from the first transmittingsurface A to pass through and exit the first prism, the partialreflecting surface B being further configured to transmit a secondportion of the passed light associated with a second spectral band ofthe plurality of infrared spectral bands towards the second prism; thesecond prism having, in sequence of light propagation from the object, afirst transmitting surface D, a reflecting surface E and a secondtransmitting surface F, the first transmitting surface D beingoperatively configured to pass the second portion of light exiting thepartial reflecting surface B of the first prism to the reflectingsurface E of the second prism, the reflecting surface E beingoperatively configured to reflect the second portion of passed lightback towards the first transmitting surface D so that the reflectedsecond portion of the passed light is internally reflected by the firsttransmitting surface D towards the second transmitting surface F, thesecond transmitting surface F being operatively configured to allow thereflected second portion of the passed light from the first transmittingsurface D to pass through and exit the second prism; a first cameradisposed along an optical path after the second transmitting surface Cof the first prism, wherein the first camera comprises a first objectivelens and a first detector, the first objective lens configured torefract the first portion of the light exiting the first prism to form afirst image at the first detector; and a second camera disposed along anoptical path after the second transmitting surface F of the secondprism, wherein the second camera comprises a second objective lens and asecond detector, the second objective lens configured to refract thesecond portion of the light exiting the second prism to form a secondimage at the second detector; wherein the first prism, the second prism,the first camera, and the second camera are configured such that theobjective lens assembly is attachable to a compound surface in the frontof a headgear, wherein the first camera has an optical axis disposedalong a first tangent to the compound surface of the headgear, and thesecond camera has an optical axis disposed along a second tangent to thecompound surface of the headgear, the first tangent and the secondtangent being oriented in a lateral direction toward the left and theright of a wearer's head, respectively.
 2. An objective lens assembly asset forth in claim 1, wherein the first prism has a dichroic coatingthat defines the partial reflecting surface B of the first prism and isadapted to reflect the first portion and transmit the second portion ofthe passed light.
 3. An objective lens assembly as set forth in claim 2,wherein the first portion of the passed light is within the long waveinfrared band.
 4. An objective lens assembly as set forth in claim 2,wherein the second portion of the passed light is within thenear-infrared band.
 5. An objective lens assembly as set forth in claim2, wherein the dichroic coating is adapted reflect a wavelength within arange of 8 μm to 12 μm.
 6. An objective lens assembly as set forth inclaim 2, wherein the dichroic coating is adapted transmit a wavelengthwithin a range of 0.6 μm to 0.9 μm.
 7. An objective lens assembly as setforth in claim 1, wherein the first prism is constructed from a thermalinfrared transmitting material having a refractive index (n₂), at awavelength of 10 micrometers, greater than 2.4.
 8. An objective lensassembly as set forth in claim 1, wherein the first prism comprises zincselenide.
 9. An objective lens assembly as set forth in claim 1, furthercomprising a bonding layer disposed between the partial reflectingsurface B of the first prism and the first transmitting surface D of thesecond prism, wherein the bonding layer is adapted to transmit awavelength within the near-infrared band.
 10. An objective lens assemblyas set forth in claim 1, further comprising a bonding layer disposedbetween the partial reflecting surface B of the first prism and thefirst transmitting surface D of the second prism, wherein a ratio(n₃/n₂) of a refractive index (n₃) of the bonding layer to a refractiveindex (n₂) of the first prism is equal to or greater than 0.650.
 11. Anobjective lens assembly as set forth in claim 10, wherein the bondinglayer comprises one of NOA-61 or NOA-71.
 12. An objective lens assemblyas set forth in claim 1, wherein the second prism is constructed from athermal infrared transmitting material having a refractive index (n₄) ata wavelength of 0.85 micrometers, greater than 2.5.
 13. An objectivelens assembly as set forth in claim 1, wherein the second prismcomprises zinc selenide.
 14. An objective lens assembly as set forth inclaim 1, wherein the reflecting surface E of the second prism has ametallic mirror coating.
 15. An objective lens assembly as set forth inclaim 1, wherein the first tangent and the second tangent are orientedat an angle with respect to each other.
 16. An objective lens assemblyas set forth in claim 15, wherein the angle is less than 180 degrees andgreater than 120 degrees.
 17. An objective lens assembly as set forth inclaim 1, wherein the headgear comprises a helmet.
 18. An objective lensassembly as set forth in claim 1, wherein the first detector comprises along wavelength infrared sensor array.
 19. An objective lens assembly asset forth in claim 18, wherein the second detector comprises a visibleand near infrared sensor array.
 20. An imaging device for simultaneouslyimaging a plurality of infrared spectral bands, comprising: a firstprism and a second prism, wherein: the first prism has, in sequence oflight propagation from an object, a first transmitting surface Adefining an entrance pupil of the objective lens assembly, a partialreflecting surface B and a second transmitting surface C, the firsttransmitting surface A being operatively configured to pass light fromthe object to the partial reflecting surface B, the partial reflectingsurface B being operatively configured to reflect a first portion of thepassed light associated with a first spectral band of the plurality ofinfrared spectral bands back towards the first transmitting surface A sothat the reflected first portion of the passed light is internallyreflected by the first transmitting surface A towards the secondtransmitting surface C, the second transmitting surface C beingoperatively configured to allow the reflected first portion of thepassed light from the first transmitting surface A to pass through andexit the first prism, the partial reflecting surface B being furtherconfigured to transmit a second portion of the passed light associatedwith a second spectral band of the plurality of infrared spectral bandstowards the second prism, and the second prism has, in sequence of lightpropagation from the object, a first transmitting surface D, areflecting surface E and a second transmitting surface F, the firsttransmitting surface D being operatively configured to pass the secondportion of light exiting the partial reflecting surface B of the firstprism to the reflecting surface E of the second prism, the reflectingsurface E being operatively configured to reflect the second portion ofpassed light back towards the first transmitting surface D so that thereflected second portion of the passed light is internally reflected bythe first transmitting surface D towards the second transmitting surfaceF, the second transmitting surface F being operatively configured toallow the reflected second portion of the passed light from the firsttransmitting surface D to pass through and exit the second prism; afirst objective lens disposed along an optical path after the secondtransmitting surface C of the first prism and configured to refract thefirst portion of the light exiting the first prism to form a first imageat a focal plane thereof; a first detector disposed at the focal planeof the first objective lens; a second objective lens disposed along anoptical path after the second transmitting surface F of the second prismand configured to refract the second portion of the light exiting thesecond prism to form a second image at a focal plane thereof; and asecond detector disposed at the focal plane of the second objectivelens; wherein the imaging device is attachable to a compound surface inthe front of a headgear such that the first transmitting surface A ofthe first prism is orthogonal to a central axis of the headgear, thefirst objective lens has an optical axis disposed along a first tangentto the compound surface of the headgear, and the second objective lenshas an optical axis disposed along a second tangent to the compoundsurface of the headgear, the central axis of the headgear being along adirection from the front to the back of a wearer's head, the firsttangent and the second tangent being oriented in a lateral directiontoward the left or the right of the wearer's head.
 21. An imaging deviceas set forth in claim 20, wherein the first tangent and the secondtangent are oriented at an angle with respect to each other.
 22. Anobjective lens assembly as set forth in claim 21, wherein the angle isless than 180 degrees and greater than 120 degrees.
 23. An imagingdevice as set forth in claim 20, wherein the first detector comprises along wavelength infrared focal plane array.
 24. An imaging device as setforth in claim 23, wherein the second detector comprises a visible andnear infrared focal plane array.